1. Field of the Invention
The present invention relates to an electroluminescent (EL) display and a process for producing the same.
2. Background Art
(1) First Aspect of the Invention
In recent years, flat displays have become used in various fields and places, and advance in information technology has rendered flat displays more and more important. At the present time, liquid crystal displays (LCDs) are representative flat displays. The development of organic ELs, inorganic ELs, plasma display panels (PDPs), light emitting diode displays (LEDs), vacuum fluorescent displays (VFDs), field emission displays (FEDs) and the like as flat displays based on a display principle different from that of LCDs are also being energetically made. All of these novel flat displays are displays called self-luminescent type. These self-luminescent displays are greatly different from LCDs in the following points and have excellent properties not possessed by the LCDs.
LCDs are displays called photoreception type. Liquid crystal per se does not emit light and functions as the so-called shutter for permitting the transmission of external light or cutting off the external light to constitute a display. Due to this nature of the liquid crystal, LCDs require the use of a light source, generally backlight. On the other hand, self-luminescent displays per se emit light, and, thus, there is no need to provide a separate light source for emitting light. In the photoreception-type displays like LCDs, backlight is always in a lighted state regardless of the mode of information displayed, and power consumption consumed in non-display state is substantially the same as that in wholly display state. On the other hand, in the self-luminescent displays, power is consumed only in sites necessary for lighting depending upon display information. Therefore, the power consumption is advantageously smaller than that of the photoreception-type displays.
Further, in LCDs, since a dark state is provided by cutting off light from the backlight, it is difficult to completely prevent light leakage. On the other hand, in the self-luminescent displays, non-luminescent state is the dark state. Therefore, an ideal dark state can easily be provided, and, thus, the self-luminescent displays are also much superior to LCDs in contrast.
In LCDs, since polarized light control by taking advantage of birefringence of liquid crystals is utilized, the display state greatly varies depending upon the direction of viewing. That is, the display state is highly dependent upon the view angle. On the other hand, self-luminescent displays are substantially free from this problem.
Further, in LCDs, since a change in orientation derived from dielectric anisotropy of liquid crystals as an organic elastic substance is utilized, theoretically, the time of response to an electric signal is not less than 1 ms. On the other hand, in the above new technology of which the development is being forwarded, electrons/holes, that is, carrier transition, electron release, plasma discharge and the like are utilized. Therefore, the response time is on the order of ns, that is, is much higher than the response speed of LCDs, and, thus, the self-luminescent displays are free from a problem of after image of moving images attributable to the slow response speed of the LCDs.
Among the self-luminescent displays, organic ELs have been particularly energetically studied. Organic ELs are also called “OEL” or “organic light emitting diode (OLED).”
An OEL element and an OLED element have such a construction that a layer containing an organic compound (EL layer) is interposed between a pair of electrodes, an anode and a cathode. A fundamental structure of this element is a laminate structure of “anode electrode/hole injection layer/luminescent layer/cathode electrode” proposed by Tang et al. (Japanese Patent No. 1526026). Tang et al. Japanese Patent No. 1526026 uses a low-molecular material. On the other hand, Nakano et al. Japanese Patent Laid-Open No. 273087/1991 uses a high-molecular material.
Further, an attempt to use a hole injection layer or an electron injection layer has been made to improve efficiency. Furthermore, an attempt to dope a luminescent layer with a fluorescent dye or the like has been made to control colors of emitted light.
The construction of an EL element is generally such that an EL layer is formed on an anode provided for each pixel and a cathode is provided as a common electrode on the EL layer. In this case, the thickness of the anode is large and is about 200 nm from the viewpoint of lowering electric resistance. The EL layer having a small thickness of 30 to 150 nm is formed on the thick anode. Therefore, disadvantageously, breaking of the EL layer occurs on the side face of the anode. Breaking of the EL layer disadvantageously causes shortcircuiting between the anode and the cathode in the broken part. This makes it impossible to exhibit luminescence of the EL layer, and black point defects are formed. In the prior art technique, when the EL layer is formed by the vapor deposition, the thickness of the EL layer in its part located at the boundary between the partition wall and the electrode is smaller than the other parts, and current concentration occurs in this part. In order to solve the problem involved in the prior art technique, that is, the problem of electrode breaking and the problem of the smaller thickness of the EL layer in its part located at the boundary between the partition wall and the electrode, in Yamazaki et al. Japanese Patent Laid-Open No. 164181/2002, as shown in FIGS. 10 and 11, the upper ends 300, 400 of tapered partition walls are convexly curved in section relative to the substrate, and the lower ends 301, 401 of the tapered partition walls are concavely curved in section relative to the substrate. This construction is described to have solved the problem of electrode breaking and the problem of uneven layer thickness.
The present inventor has made experiments using partition walls proposed in the Yamazaki et al. publication. As a result, it was confirmed that the problem of electrode breaking did not occur. However, when the EL layer was formed by an ink jet method, as shown in FIG. 12, the problem of uneven layer thickness became more significant. The reason for this is probably that a liquid reservoir phenomenon occurs in the concavely curved part in the lower end 301 and, due to this phenomenon, attraction of the EL layer ink to the side face of the partition wall is enhanced.
The formation of the EL layer by a wet process has many advantages and is a promising method for the preparation of an organic EL display. However, except for the following complicated Inoue's process, there was no method for evenly controlling the layer thickness. A well-known method for the preparation of an organic EL display is one described in Inoue: “Kara Porima EL Disupurei (Color Polymer EL Display),” Vol. 22, No. 11, O plus E, p. 1433-1440. In this method, as shown in FIG. 5, partition walls 4 are formed on an insulating layer 8. An luminescent material 5 in an ink form is ejected through an ink jet nozzle 9 to put the luminescent material 5 selectively on a pixel opening 6 (FIG. 15). In order to fix the ink of the luminescent material, the pixel opening and the insulating layer are treated to impart hydrophilic nature. The insulating layer is provided for preventing insulation failure between the opposed electrodes caused by electric field concentration at the edge of the electrode, that is, interelectrode leak. Further, as shown in FIG. 5, the partition walls are subjected to water repellency-imparting treatment so that ink droplets not ejected on the pixel opening but impacted on the partition wall are allowed to flow in the pixel opening.
Fujita et al. Japanese Patent Laid-Open No. 351787/2001 proposes an organic EL element having partition walls similar to the partition walls described in Yamazaki et al. document. The partition wall has a foot part having a triangle-, trapezoid- or arc-like tapered shape around the electrode, and the foot part is concavely curved. The EL layer is formed by a printing method. What is referred to in the Fujita et al. publication is electrode breaking at the lower ends 301, 401 in the Yamazakils display, and the Fujita et al. publication is silent on the evenness of the layer thickness. FIGS. 2(a), (b) and (c) in Japanese Patent Laid-Open No. 351787/2001, however, shows the bulged shape of the EL layer along the side face of the partition wall. Thus, the problems to be solved by the present invention remain unsolved.
Under these circumstances, the present invention has been made, and an object of the present invention is to provide an organic electroluminescent display having excellent practicality that can be produced by a simple method which does not cause electrode breaking and can realize even thickness of an electroluminescent layer.
(2) Second Aspect of the Invention
An electroluminescent (EL) element includes a pair of opposed electrodes. A luminescent layer containing an organic fluorescent coloring matter, optionally together with other layers such as a hole injection layer, is interposed between the pair of electrodes. In this EL element, upon recombination of electrons and holes provided in the luminescent layer, energy is generated. The energy excites the phosphor in the luminescent layer to emit light. In the EL element, a reduction in thickness and a reduction in weight can be realized. Further, the EL element has high brightness and is also suitable for the display of moving images. Therefore, the development of EL elements for various display applications have been forwarded.
In a general EL element, in general, three types of luminescent layers different from one another in color of luminescence should be regularly arranged. To this end, various methods have been studied. A currently commonly employed method is to form the luminescent layer by an ink jet method using a coating liquid for luminescent layer formation. In order to prevent the formation of the luminescent layer in an area other than the predetermined area, a partition wall is provided between adjacent pixels so that the coating liquid for the formation of a luminescent layer, which exhibits luminescence of a specific color, is applied only in the inside of a space surrounded by the partition walls.
Japanese Patent Laid-Open No. 323276/2000 discloses a pattern formed object as shown in FIG. 27. Specifically, a partition wall 1014 formed of a photosensitive polyimide or the like is provided between pixel electrodes 1013 of ITO film or the like provided on a transparent substrate 1012. The assembly is subjected to continuous plasma treatment of oxygen gas plasma and fluorocarbon gas plasma to render the surface of the electrode hydrophilic and to render the surface of the partition wall 1014 water-repellent. Both a hole injection layer and a luminescent layer are formed by an ink jet method to form a pattern formed object (an element) 1011. Since, however, the surface of the partition wall 1014 of polyimide has been rendered water-repellent, upon the application of the coating liquid by the ink jet method, the partition wall 1014 repels the coating liquid 1015. Therefore, as shown in FIG. 27A, the application of the coating liquid 1015 in its center part is in a bulged state. Thus, a coating (a luminescent layer) 1015′, in which the center of the pixel is in a bulged state, is formed. In this coating (luminescent layer) 1015′ (FIG. 27B), the concentration of an electric field occurs in a relatively thin layer part. This leads to a drawback that only the peripheral part of the luminescent layer along the partition wall exhibits luminescence.
Japanese Patent Laid-Open No. 351787/2001 discloses that the part around the partition wall, in which the luminescent layer is not formed, can be significantly reduced by providing partition walls having a tapered foot part in which the surface of the foot part is concavely curved in section. In the formation of the partition walls, however, although only some description on materials for black matrix (chromium and resin black are described as an example; and a resist is used in the working example) is found, there is no specific description on the formation of the unique shape in the foot part of the partition wall. Therefore, reproduction of the partition wall is difficult.
Japanese Patent Laid-Open No. 148429/2002 discloses that, when the partition walls are subjected to both roughening treatment (to provide Ra about 3 to 50 nm) and plasma treatment using fluorine element-containing gas, the ink adhesion can be improved by the former treatment to reduce the level of bulge and, thus, it is possible to provide such a sectional form that the level of bulge of the center part is small, the height of the peripheral part is high, and the layer thickness other than that of the peripheral part is uniform. However, in addition to the fact that the bulge in the center part is not fully removed, the height of the coating at the peripheral part becomes large. Therefore, a part, located somewhat inward from the peripheral part, has a smaller layer thickness. Further, since the maximum layer thickness and the minimum layer thickness are within ±25% of the average layer thickness, disadvantageously, for example, only a part near the partition wall exhibits luminescence, or otherwise luminescence is exhibited only in a narrowed region.
Further, in the above methods described in Japanese Patent Laid-Open Nos. 323276/2000 and 148429/2002, since the partition walls are liquid-repellent, a hole injection layer, which is in many cases provided together with the luminescent layer, is less likely to be evenly formed. Unlike the luminescent layer, in the formation of the hole injection layer, there is no need to change color of luminescence for each pixel, and the hole injection layer may be formed on the whole area.
Accordingly, an object of the present invention is to provide a pattern formed object having a coating as an electroluminescent layer or the like having even thickness, for an electroluminescent element, formed using a coating liquid and provided in each area surrounded by partition walls provided on a substrate provided with partition walls.